In general, the present invention relates to methods, devices, and systems for providing position, velocity and time information in all types of weather, such as the satellite navigation system known as NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Positioning System--herein, GPS), and more particularly, the invention relates to a new method, new computerized signal receiving device, and new system for determining an optimal value for an integer number of phases or cycles of a signal wave being transmitted from a distant source. In Global Positioning Systems, the integer number of phases, or cycles/wavelengths in the frequency domain, of the carrier signal wave(s) that stretches between the satellite and receiver is referred to as the "integer ambiguity" (or simply "ambiguity"). The ambiguity must be resolved before a position of the receiver can be determined. Although the discussion of the novel method, device, and system of the invention will focus on methods and components of the popular, well developed GPS, the invention has wider applications and need not be limited as such.
The GPS has been used for years as a tool for surveying, strategic military locating, and more-recently in commercial and private vehicles and aircraft navigation. As is well known, GPS satellites transmit two L-band pure sinusoid signals ("carrier" signals L.sub.1 and L.sub.2) into which a course/acquisition, or clear/access (C/A), pseudo random binary code and a precise (P) pseudo random binary code are modulated. This modulation, or alteration, of the pure sine wave signals is done to permit time delay measurements to be made for positioning purposes. The C/A and P codes consist of sequences of binary values (zeros and ones) generated by hardware devices called tapped linear feedback registers which are capable of generating a sequence of ones and zeros that does not repeat during some particular chosen interval of time. An appropriately equipped GPS receiver can independently locally replicate these code sequences and align the replicated sequence with the corresponding sequence contained in the received signal. Then, by knowing the instant of time the code sequence was transmitted, the signal's travel time can be determined so that the range from receiver to satellite can be computed. Even though all the signals transmitted from the several GPS satellites utilize the same frequencies, each satellite has its own unique identifier code so that a receiver can distinguish between signals received from different satellites.
The fundamental clock frequency, "f.sub.o ", is 10.23 MHz: This is the frequency at which the two L-band carriers are transmitted from each satellite. The L.sub.1 carrier has a frequency of 1575.42 MHz and a wavelength of approximately 19 cm. The L.sub.1 carrier is modulated by the C/A code which consists of 1023 binary chips transmitted at a frequency that is one-tenth of f.sub.o so that it repeats every millisecond. The L.sub.2 carrier has a frequency of 1227.60 MHz and a wavelength of approximately 24 cm. Since the Anti Spoofing (AS) feature of the GPS signal was activated (January 1994), the P code has been encrypted and is no longer accessible to unauthorized users who must rely on codeless technology to obtain L.sub.2 carrier phase measurements. Therefore, as will be better appreciated, it is desirable to eliminate the need to rely on P code and/or the L.sub.2 carrier phase measurements for determining a receiver's position.
Two Types of GPS Measurements: A code tracking measurement (the so-called "pseudorange measurement") is obtained by correlating the C/A modulation of a received signal with a corresponding sequence generated in a receiver. The code measurement equals the observed difference in time between a satellite clock and the receiver clock generating the local modulation sequence. Because the C/A code is repeated so often, a receiver can quickly lock onto a received signal and begin matching the received code with the corresponding code generated within the receiver. Another type of measurement that can be taken from GPS satellites is a carrier phase measurement where the relative phase is measured between the received, reconstructed carrier phase and the receiver clock phase at a particular epoch (i.e., defined measurement period--ranging from an hour or more to a fraction of a second): For reference, see FIG. 1 schematic.
FIG. 1 (at 10) represents a positioning system such as the popular GPS. GPS code measurements are substantially noisier than the carrier phase measurements 18. And, although prone to receiver and satellite biases and errors (as well as tropospheric and ionospheric delays), a carrier phase measurement can give more-accurate measurements of the change in the pseudorange with time and of the fractional wavelength of the pseudorange distance to the satellite. When a GPS receiver (FIG. 1 at 12) locks-on to a GPS satellite signal 16, the initial phase measurement is biased by an arbitrary number of whole cycles of the carrier frequency that is received by the antennae. The receiver has no way of determining just how many complete wavelengths (see FIG. 1 where the "integer ambiguity" is denoted as "n" at 20) are contained in the electromagnetic signal 16 stretching between receiver 12 and satellite 14. It is well known that, when taking carrier phase measurement, a value for the integer ambiguity must be determined (resolved) accurately so that the measured phase can be converted into a precise range (or distance) to the satellite. This, in turn, allows for accurate identification of a receiver's position. It is desirable to identify a position by, for example, giving its geographic coordinates or UTM coordinates which can then be mapped onto an electronic display showing the receiver's location within a city or region (such as a mountain range, desert region, and so on).
Under ideal conditions, the integer ambiguity can be resolved quickly. However, under ordinary, everyday circumstances biases and errors resulting from receivers and satellites, as well as tropospheric and ionospheric delays, make determination of the ambiguity (which tends to fluctuate with time) very complicated. Even traditional, well-known static differential GPS positioning, where the receiver remains in a fixed spot and is able to spend a relatively long time (i.e., an epoch of up to an hour or two) collecting uninterrupted phase measurements to isolate an integer ambiguity for a particular receiver-to-satellite range, requires significant processor computation time and effort.
The traditional static differential GPS positioning has been supplanted by more-productive so-called kinematic surveying, where the receiver is considered "in-motion" and allotted much less time at any one position to collect phase measurement data (an epoch of one minute or less). Kinematic differential positioning uses carrier phase observations or measurements to resolve the integer ambiguity--this is commonly referred to as "on-the-fly" determination of ambiguities. Known on-the-fly techniques can be lumped together as being deterministic in nature. That is to say, known GPS on-the-fly techniques resolve the ambiguity by testing many combinations of ambiguity sets that fit within a certain predefined and specifically constructed mathematical search space (defined in the ambiguity domain) or a predefined and specifically constructed physical search space (defined in a position domain). Although construction approaches of the predefined search space vary in known on-the-fly techniques, none take advantage of the wealth of information embedded within the GPS measurements collected.
Most such known on-the-fly techniques require the use of the more-expensive dual-frequency receivers with stable carrier phase tracking loops. However, these dual-frequency receivers must rely on a codeless correlation technique(s) to reconstruct the L.sub.2 carrier signal. For example, Trimble Navigation Ltd.'s model 4000 SSE/SSi.TM. receiver uses a cross-correlation technique while Ashtech, Inc.'s model Z-XII/P-12 receiver employs a Z-TRACKING.TM. technique to reconstruct the full L.sub.2 carrier wavelength. These L.sub.2 carrier reconstruction techniques remain unproven under many operational conditions and have substantial degradation in the signal to noise ratio making it difficult to resolve ambiguities even when using these sophisticated dual-frequency receivers.
Hans Euler, in his 1994 IEEE paper presented at the Position Location and Navigation Symposium ("Achieving High-Accuracy Relative Positioning in Real-time: System Design, Performance and Real-Time Results"), reports of his success using real-time processing software on a Leica AG's WILD CR244.TM. GPS handheld controller (INTEL.RTM. 386-chip) that uses a recursive least-squares algorithm with pre-elimination of unknowns used for one epoch and single-difference modeling of the observations. During the initialization period, the algorithm delivers the actual computed position. The GPS sensor used was the Leica AG's WILD SR299.TM., a dual-frequency receiver with 9 channels for the L.sub.1 phase and 9 channels for L.sub.2 phase tracking that delivers 4 independent measurements (2 pseudo ranges and 2 phases) for every satellite. The pseudo ranges are measured using the C/A code on L.sub.1 and the P code on L.sub.2. If Anti-Spoofing is turned on, the pseudo range on L.sub.2 is measured by using a proprietary P code aided tracking technique (see discussion of reconstruction of the L.sub.2 carrier, above).
U.S. Pat. No. 4,812,991 issued to Hatch (1989) discloses a method and apparatus for determining the position coordinates of a remote, movable receiver relative to a fixed reference receiver that uses successive code measurements and carrier phase measurements of both the L.sub.1 and L.sub.2 carrier signals broadcast from four or more orbiting GPS satellites. Code measurements based on a weighted average of the individual L.sub.1 and L.sub.2 code measurements in each satellite/receiver link are adjusted in accordance with the corresponding carrier phase measurement for an L.sub.1 and L.sub.2 carrier difference signal and are further smoothed over time. It takes at least two to three minutes of processing to yield a position determination that is accurate to about 1 centimeter (cm).
U.S. Pat. No. 4,963,889 issued to Hatch (1990) discloses a complex time-consuming technique for resolving whole-cycle ambiguity. The relative position of a secondary receiving antenna with respect to a reference antenna is approximately known or approximately initially determined and then measurements from a minimum number of satellites are used to determine an initial set of potential solutions to the relative position of the secondary antenna that fall within a region of uncertainty surrounding the approximate position. Redundant measurements are taken from one or more additional satellites and used to progressively reduce the number of potential solutions to close to one. Even if the number of potential solutions is not reduced to one true solutions, the number can be further reduced by using additional measurements taken at different time intervals, at which different satellite geometries prevail.
Caroline Erickson in her "Analysis of Ambiguity Resolution Techniques for Rapid Static GPS Surveys Using Single Frequency Data" analyzes and compares three ambiguity resolution techniques (all based on double-difference observations) for rapid static GPS surveys over short baselines using single frequency data with C/A code measurements: the Ambiguity Function Method (AFM), the Fast Ambiguity Resolution Approach (FARA), and the Least Squares Ambiguity Search Technique (LSAST). She reports that the AFM differs from least squares techniques because trial positions are searched instead of trial ambiguity sets, which results in the unique property of AFM being invariant to cycle slips.
Patrick Y. C. Hwang in his 1990 IEEE paper "Kinematic GPS: Resolving Integer Ambiguities On The Fly" proposes two ideas for adapting standard kinematic techniques to situations that do not naturally allow for the constraint of a fixed baseline. The first calls for extracting the information needed to resolve the integer ambiguity from the data collected while the kinematic survey is in progress. The second idea addresses the use of the antenna exchange technique for mobile platforms where the original locations of the antennas are not likely to remain stationary during the physical exchange. Both ideas count on information from additional measurements to augment their respective measurement models.
It can be appreciated that, due to encryption and general unavailability of the P code and resulting unavailability of a direct L.sub.2 carrier signal (reconstruction being necessary), plus the need/desire to cut computation time of the ambiguity resolution, known and currently available GPS static and kinematic positioning techniques are very limited in use. A robust on-the-fly GPS technique is needed that only has to rely on L.sub.1 carrier phase measurements, as derived from the publically-accessible C/A code, using a single-frequency signal receiver (but could be used with an L.sub.1 /L.sub.2 dual-frequency receiver). The innovative method, computerized device and system, described herein, has a wide range of static and kinematic (where the receiver only has a minute, or a few seconds, at any particular position to take measurements) positioning applications.
The innovative method of determining a value for an integer number of signal phases/cycles/wavelengths using the computerized device described herein, can collect the necessary data in a very short period and rapidly resolve an integer ambiguity without requiring substantial hardware changes to the components of known positioning systems Code for carrying out the method of the invention can be readily drafted and installed onto currently available positioning receivers having currently-available computer processing units (CPU) similar to those found in personal computers (PCs), workstations, laptop PCs, and handheld devices such as palmtop PCs.
Unlike the GPS devices and techniques currently available for resolving the integer ambiguity, the new method, device, and system were developed to utilize processing time more efficiently while at the same time provide sufficient or optimal solutions to resolution of the integer ambiguity without requiring close initial estimates of the ambiguity. More particularly, unlike known GPS methods and devices used for resolving the integer ambiguity, true initial ambiguity values are not required to resolve the integer ambiguity for positioning. In fact, initial ambiguity values need not be very close at all to optimally resolve the integer ambiguity using the new method, device, and system.
Additionally, unlike known GPS mathematical techniques developed such as Ambiguity Function Mapping method, AFM, (which, alone, relies on a deterministic trial and error procedure to compute each corner of a cubed search volume in an attempt to find a unique set of integer ambiguities), the new method, device, and system apply a collective learning process using a random, or probabilistic, search technique. As will be better appreciated, the flexible method, device, and system of the invention can be: incorporated into currently available GPS orbiting satellite transmitters and kinematic or static signal receiving hardware located on earth; used with transmitters and receivers that are either both in orbit or both located within the earth's atmosphere (such as on an aircraft); used with transmitters and receivers that are both located on earth (for example, the transmitter located on "high ground" such as on a hill top or on a mountain and the receiver installed in a traveling vehicle); and so on, in the spirit of the design goals for the instant invention.